How can I add a second texture apart from the one I'm using with displacement modifier?
Yes of course you can still have different textures when using cycles. However that way you add them, and access them is a little different.

When you are using cycles you no longer start your texture in the texture tab of the properties window (like how you had to do it for particles), now much like the displace modifier there is a button to jump right to your texture.  



Now for the particle system, you do much the same thing as for the displace modifier.


Go to the particle tab and add a new texture.

Click on the "Show texture in texture tab" button. (You could also go to the texture tab your self, more on that in a bit.)
You are now editing the texture for the particles, set what ever influences you want.






The texture tab works a little differently then when you are using the blender internal.
In cycles all the textures are nicely broken up by category, and you can edit any of them at any time.


While you can make new textures and assign them to different uses (say I make a new texture for the displace modifier, I can switch right in the texture tab.)
However you can not add new texture uses from the texture tab. So if you do not already have the displace modifier using a texture you can not assign a texture to it in the texture tab.